Transmitting power in a vehicle generates a great amount of heat within a generally enclosed and rigid housing. The substantially hollow interiors of these housings, which include axles, transmissions and differentials, contain lubricants and air. The lubricants do not expand or contract to any appreciable extent due to the temperature changes, but the air present in the housing reacts much more strongly to the differences. Since the walls of the housing are rigid, pressure differences between the interior and exterior of the housing can put undue stress on gaskets and other sealing devices which are used with adjoining pieces of the housing if pressure differences cannot be relieved. To prevent failure of these sealing devices, accompanied by loss of lubricant and possible introduction of contaminants into the housing, it is desirable and known to communicate the interior of the housing with the exterior. As heat is generated, excess pressure in the housing may be vented to the exterior and, upon cooling, makeup air may be drawn into the housing. Such an air bleed device needs to selectively allow passage of air rather than lubricant.
While air bleed devices are known for vehicle power transmission housings, there is clearly room for improvements in terms of simplicity, compactness, and various structures which protect the vent from damage and from outside contaminants.